Imaging has become one of most important techniques in identifying and monitoring cancer in non-invasive diagnosis as well as in accurate surgical resection during treatment. First, it is crucial to detect tumor cells at an early stage of development. The most commonly used methods for detecting cancer in patients include magnetic resonance imaging (MRI), computerized tomography (CT), and positron-emission tomography (PET). Second, cancer treatments can include surgical resection, chemotherapy, and radiation. Surgery is one of the most effective ways to remove tumors and avoid metastatic disease spread. Surgery cures approximately 45% of all cancer patients with solid tumors. To be considered successful, a surgeon must remove the entire tumor and any lymph nodes or satellite nodules containing tumor cells. Partial removal of the solid tumor and incomplete removal of all the tumor cells decrease a patient's survival rate by 5-fold. Therefore, it is important to map the solid tumor accurately using imaging before surgery and double check for any residual tumor cells during and after surgery.
Recently, fluorescence imaging has been suggested for use in both cancer detection and resection; however, it is a light-dependent method. Fluorescence imaging requires an external light source to excite exogenously-added fluorescence agents and is not very versatile due to the fact that many biological molecules present in the body have significant absorption of wavelengths at both visible and infrared regions of the light spectrum. Further, many wavelengths generated from the external, excitation light source cannot penetrate tissues to reach the imaging fluorescent molecules, especially when they are in solid tumors. When the fluorescent molecules are not excited, no light is emitted for detection of the cancer. To overcome this problem, luminescence imaging is being developed to emit light from within the solid tumor. The process involves an enzyme, such as luciferase, which catalyzes the oxidation of a substrate, i.e., luciferin, to generate a bioluminescent signal, which is measurable by photon emission. Light is generated without application of an external excitation light source. The enzymatically-generated photons are able to travel through solid tumors during the detection process.
Although humans and animal models of cancer do not have naturally occurring bioluminescent genes, such as luciferase, the genes or proteins can be introduced for imaging purposes. For instance, bacteria encoded with a luciferase gene can accumulate in C6 glioma tumors in a mouse model. The expressed luciferase is then used for cancer detection in vivo. Furthermore, mammalian cells genetically modified with a luciferase gene can be delivered directly into tumors in live animals for tumor detection using bioluminescence.
In addition, tumor bioluminescence can be a useful tool during surgery as shown in an animal model. Bioluminescence was able to precisely detect both tumors preoperatively and intra-operatively. For the enzyme to produce bioluminescence within the tumor, D-luciferin or other small molecule substrates, such as coelenterazine, vargulin, and 6-aminoluciferin derivatives, must be administered. However, bioluminescence generated from these substrates lasts for a very short time—only 15 to 20 minutes per administration. This is not practical in a clinical setting as the surgery to remove a brain tumor takes 3 to 5 hours. Checking and re-checking for tumor remnants by bioluminescence would require multiple new administrations of substrate. The additional application steps would lengthen the surgical time and potentially complicate the surgery and reduce the success rate of the surgery. Therefore, a new substrate with longer lasting bioluminescence signal is needed.
A need, therefore, exists in the art for new probes for the detection of cancer cells.